1. Field of the Invention
The present invention relates to a mobile communications system and, more particularly, to a method and system for controlling mobility management in a mobile communications system, as well as to a base station in the mobile communications system.
2. Description of the Related Art
A mobile communications network in general is provided with a function called mobility management (MM) for managing the location of a mobile station (or UE: User Equipment), with which it is possible to track the location of a mobile station and to page the mobile station even if the mobile station is in an idle state of not carrying out communication. The tracking of the location of a mobile station is performed in at an area granularity composed of a plurality of cells, which is called a location management area or tracking area (hereinafter, abbreviated as “TA” where appropriate). Location management is performed in such a manner that a mobile station is assigned a temporary unique number (temporary identity) in a tracking area in which the mobile station is currently registered. When the mobile station moves to another tracking area, the location management of the mobile station is performed using a temporary identity that is assigned to the mobile station in that new tracking area (see 3GPP TR 23.822 V0.10.0 (2006-01), Technical Report, pp. 29-33).
FIG. 1 is a schematic diagram of a conventional mobile communications system. Here, to avoid complexity, it is assumed that a tracking area 7 is composed of cells 4 to 6, which are covered by three base stations (BTS) 1 to 3 respectively, and that a tracking area 10 is composed of a cell 9, which is covered by a base station 8. These base stations 1 to 3 and 8 can communicate with each other by connecting to a router 11 through transmission paths Xub1 to Xub4 respectively and, via the router 11, are further connected to a BTS controller 12 and a gateway (GW) 13.
In such a conventional system, a mobility management entity MME exists in a central controller such as the gateway 13 or BTS controller 12 and performs location management such as assigning a temporary identity to a mobile station. For example, as shown in FIG. 1, when a mobile station 14 has moved from the tracking area 10 into the tracking area 7, the mobile station 14 knows from a broadcast signal received from the base station 1 that the mobile station 14 has come into the new tracking area 7. Upon knowing of this, the mobile station 14 sends an area update request to the base station 1 and is assigned a temporary identity by the gateway 13 so that location management is carried out in the tracking area 7. The mobility management for the mobile station 14 is performed by the gateway 13 through the base station 1 to which the area update request is first sent. When the mobile station 14 is in idle mode, it periodically searches for a best radio cell to camp on. Such a best radio cell is often called as “camped cell” and the base station controlling the “camped cell” is noted as “camped base station”.
For example, when a packet destined for the mobile station 14 arrives at the gateway 13 from the Internet 15, the gateway 13 inquires of all the base stations in the tracking area 7 where the mobile station 14 is located. Thereby, it is found which base station currently accommodates the mobile station 14, and the gateway 13 can transfer the packet destined for the mobile station 14 to that base station.
In the above-described conventional system, the mobility management entity MME exists in the central controller such as the gateway or BTS controller. However, it is also possible to equip each of the plurality of base stations with the same mobility management function. In general, there is a trend to relocate management functions, such as the mobility management function, from a central controller to an end base station.
However, in a mobility management system in which each base station is provided with the mobility management function, the loads of mobility management processing are uneven among the base stations. A base station that is installed near the boundary of a tracking area in particular will accept location management requests from terminals newly coming into the tracking area, and therefore the problem arises that the load on this base station might be larger than those on other base stations.
FIG. 2 is a schematic diagram showing an example in which an excessive load state occurs in a distributed mobility management system. It is assumed that each base station in a tracking area TA is provided with a mobility management function and that a base station BTS1 is installed near the boundary of the tracking area TA. If a large number of mobile stations come into the tracking area TA at a time by, for example, being transported by train, the base station BTS1 will have to process new area update requests from these many mobile stations. In this case, the following two problems may arise.
1) The first problem is that the temporary identities (unique numbers) may be depleted that are to be assigned individually to the mobile stations by the base station BTS1 positioned near the tracking area boundary. In general, the number of temporary identities for mobile station identification assigned within the tracking area TA is predetermined, and this predetermined number of temporary identities is allocated among the base stations in the tracking area TA evenly, or depending on the circumstance of each base station. Therefore, the number of temporary identities for mobile station identification allotted to the base station BTS1 is finite. Accordingly, if a large number of mobile stations send new area update requests at a time, the possibility increases that the temporary identities to be assigned to these mobile stations are depleted. If a mobile station is not assigned a temporary identity, this mobile station cannot be identified in this tracking area TA, resulting in location management being impossible. As a matter of course, when a packet destined for this mobile station arrives at a gateway GW, the packet cannot be transferred to this mobile station.
2) The second problem is that a new location management request cannot be processed because of the increased amount of processing in the base station BTS1 positioned near the tracking area boundary. As mentioned above, the base station BTS1 serves the large number of mobile stations as their respective serving base stations and has to transmit and receive control signals for location management to/from the gateway GW. Therefore, the amount of processing in the base station BTS1 tends to increase with the passage of time, and when the amount of processing reaches the limit of the throughput of the base station BTS1, the base station BTS1 cannot deal with a new area update request. Additionally, as the amount of processing in the base station BTS1 increases, the rate of use of a transmission path Xub1 that connects the base station BTS1 and a router also rises and may come close to the upper limit of the capacity. As a result, the possibility is high that the tracking of the location of a mobile station as described above becomes impossible, leading to a disconnection of communication with this mobile station.
FIG. 3 is a sequence diagram showing an example of a process of disconnecting communication with a mobile station in a conventional mobility management method. The mobile station 14 first receives TA information from the base station 8 in the tracking area 10 (time T1) and, in response to this, makes a TA update request (time T2). The base station 8 sends a serving BTS (MMSBTS) update request to the gateway 13 (time T3) and, when receiving an update ACK response to this request from the gateway 13 (time T4), sends a TA update ACK response to the mobile station 14 (time T5). Hereinafter, consideration will be given of the case where the mobile station 14 registered in the tracking area 10 as described above has moved into the cell 4 of the base station 1 installed near the boundary of the tracking area 7.
Here, it is assumed that the base station 1 is short of temporary identities for mobile station identification, or the amount of processing in the base station 1 has reached the upper limit of the throughput (i.e., the base station 1 is short of a location management resource). In such a situation, when the mobile station 14 has moved into the cell 4, the mobile station 14 first receives TA information from the base station 1 in the tracking area 7 (time T9). In response to this, the mobile station 14 sends a TA updated request (time T10). However, the base station 1 is not able to accept the TA update request from the mobile station 14 and therefore sends back a TA update NACK response (time T11). Accordingly, the mobile station 14 remains area-registered with the base station 8 in the tracking area 10 although the mobile station 14 is already in the cell 4. In this state, when a packet destined for the mobile station 14 arrives at the gateway 13 (time T7), the gateway 13 inquires of the serving base station 8 of the mobile station 14, about where the mobile station 14 is located (location search, time T8). However, the base station 8 cannot find the mobile station 14 because the mobile station 14 has already left the tracking area 10. If the base station 8 cannot find the destination (mobile station 14) even when a predetermined period of time has passed, the base station 8 responds to the gateway 13 with a NACK response to the location search request (time T11). Accordingly, the gateway 13 discards the packet destined for the mobile station 14.
As described above, the fact that a large processing load is imposed on one particular base station in a tracking area is a great factor for the inability to track a mobile station and for a disconnection of communication with the mobile station. In addition, the fact that the loads are uneven between a base station installed near a tracking area boundary and another base station not installed near the tracking area boundary means that some base stations are short of the finite number of allocated temporary identities and some base stations have a surplus. Accordingly, as a whole, the temporary identities cannot be used efficiently.
Incidentally, like the bandwidth of a transmission line and the like, the temporary identities for mobile station identification provided to a base station and the throughput of a base station can be thought of as shared resources for the mobility management of a plurality of mobile stations and the communications thereof. Therefore, hereinafter, the temporary identities for mobile station identification and the throughput of a base station will be referred to as “location management resources”, and the rate of use of any of these resources will be referred to as “location management resource use rate” or “load” as appropriate.